The present invention relates to a high efficiency light source and, in particular, to a high efficiency cold cathode gas discharge lamp control and power circuit.
A recurring problem in lighting technology is that of maximizing the efficiency and minimizing the power consumption of a light sources while meeting user needs and desires, such as simplicity, ease and low cost in fabricating and installing the light sources and, for example, in controlling the level of light output of a light source. Other factors may include, for example, the ability to place the light source where desired, to direct the light output where desired, the ability to control or determine the color of the light, and achieving an esthetic appearance of the light and light source under a range of conditions, such as when the light is dimmed as well as at full brightness.
The two most common light sources are incandescent lights and fluorescent lights. As is well known, incandescent lamps generate light by resistance heating of a filament in a vacuum or inert gas environment to such a temperature that the filament emits visible light. Incandescent lamps generally meet many of the above requirements and needs, except for inherently high power consumption and low power efficiency. Cold cathode gas discharge lamps offer much higher operating efficiencies, but generally fail to meet others of the needs and requirements outlined above.
Cold cathode gas discharge lamps, also referred to as cold cathode gas discharge tube or, because of their generally tubular shape, or as flourescent lights or tubes, emit light by the spontaneous decay of energized gas atoms excited by an externally supplied electrical discharge. As described above, the advantages of cold cathode gas discharge lamps are high output efficiency for a given power input and reduced heat generation. Gas discharge lamps also generally generate a homogenous light output over a continuous surface, thereby providing a generally more comfortable and pleasing lighting effect. In addition, different colors or emission spectrums may be readily achieved by the use of different phosphors, so that cold cathode discharge lamps, most of which generate a xe2x80x9ccoolxe2x80x9d spectrum light, can also be designed to emit, for example, a warmer light spectrum emulating daylight or light optimized for growing plants.
The disadvantages of cold cathode gas discharge lamps of the prior art, however, include the requirement for expensive, bulky and inefficient power conditioning because of the operating characteristics of the lamps. That is, cold cathode gas discharge lamps require a high initial triggering potential across or through the tube to initially excite the gas atoms into the light emitting state. Once triggered into the light emitting state, however, the gas plasma demonstrates a negative resistance characteristic wherein the resistance of the gas decreases as the discharge current through the gas increases. The negative resistance characteristic may thereby result in a runaway condition that may destroy the lamp unless the current discharge through the tube is limited. The current through the gas, after the initially triggering, must be sufficient to sustain ignition and emission of the light by the gas. A cold cathode gas discharge lamp therefore requires additional control circuits that provide both a high initial triggering potential across the tube to initiate light emission by the gas and a large current controlling impedance, referred to as a xe2x80x9cballastxe2x80x9d, to limit the current through the gas to the sustaining current level after emission is initiated. The control circuits for cold cathode gas discharge lamps are expensive, cumbersome and heavy, particularly at the conventional line frequency of 60 Hz. These problems are further compounded in that the cold cathode gas discharge lamp control circuits of the prior art typically use inductive components in the current limiting ballast circuits, which may result in a high apparent power consumption due to uncorrected power factors, particularly at 60 Hz. These disadvantages of the prior art in cold cathode gas discharge lamps and lamp control circuits have as a result largely offset the above discussed advantages of cold cathode gas discharge lamps.
The present invention is directed to a cold cathode gas discharge lamp unit includes a control circuit for controlling the light emission of the cold cathode gas discharge lamp. According to the present unit, a lamp unit includes a dc power source and a control circuit for converting dc power from the dc power source into a high frequency drive signal across the lamp wherein each cycle of the drive signal includes an ignition period with a signal level sufficient to initiate gas conduction, a sustaining period with a signal level sufficient to sustain gas conduction, and an off period with a signal level below the sustaining level.
The control circuit of the present invention includes a transformer having a primary winding connected from a dc power source and in series with a switching transistor and a secondary winding connected across the lamp and a drive signal timing circuit wherein the drive signal timing circuit includes circuit timing a feedback winding of the transformer connected between the base of the switching transistor and a timing control output of a resistor-capacitor ramp generator.
In each cycle of the drive signal, and during the off period, the drive signal timing circuit generates a timing control output having a voltage level increasing with time until the timing control output reaches a base-emitter turn on voltage of the transistor. During the ignition period current flows through the transistor and in the primary winding, the current flow being initiated by the timing control output and sustained by feedback from the primary winding to the feedback winding, until the transformer saturates and the feedback signal to transistor is terminated, driving the transistor into the non-conducting state, and a magnetic field in the transformer collapses, inducing a drive signal in the secondary winding having a signal level sufficient to initiate gas conduction in the lamp. During the sustaining period continued collapse of the magnetic field induces a drive signal in the secondary winding having a signal level sufficient to sustain gas conduction, and, during the off period, the magnetic field has collapsed and the induced drive signal in the secondary winding is at a signal level below the sustaining level.
In various embodiments of the lamp unit, the dc power source may an ac to dc adaptor connected from an ac power source and the resistor-capacitor ramp generator of the drive signal timing circuit may include a variable resistor to select the period of the drive signal and thereby to control the level of light emission.
In other embodiment, the lamp units may be incorporated into a lighting system wherein the dc power source of at least one of the lighting units may be a central dc power source, or, in an alternative embodiment of the system, an dc adapter may be located at each of one or more of the lighting units to provide dc power to those lighting units and the dc adapter may be connected from an ac power source.
In other embodiments, a lamp unit may include a tubular lamp housing with a U-shaped cold cathode gas discharge lamp mounted in the housing with all electrodes of the lamp located at a base end of the housing. The control circuit may then be mounted in the base end of the housing to convert dc power from a dc power source into the high frequency drive signal across the lamp. The lamp unit may also include a mounting bracket for attaching the light unit to a support, an dc adapter located at the lighting unit and connected from an ac power source to provide dc power to the lighting unit, and at least one canopy mounted to the tubular housing to direct the emitted light.